DETECTOR

• Main Author: WILLIAM J. MALINOWSKI
• Format: Patent
• Language: English
• Published: 18.03.1976
• Subjects: ALARM SYSTEMS; COLORIMETRY; INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIRCHEMICAL OR PHYSICAL PROPERTIES; MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT,POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRA-RED,VISIBLE OR ULTRA-VIOLET LIGHT; MEASURING; ORDER TELEGRAPHS; PHYSICS; RADIATION PYROMETRY; SIGNALLING; SIGNALLING OR CALLING SYSTEMS; TESTING

1490866 Smoke detectors CHLORIDE BATTERIJEN BV 22 Oct 1974 [26 Nov 1973 8 March 1974] 45706/74 Heading G1A A device responsive to a predetermined situation e.g. the presence of smoke, has a pulsed light source and a light detector whose output is connected to a threshold circuit such that a signal is delivered to an alarm only if the detector output exceeds a given level. The light source is pulsed to that it is typically emitting only for 20Á secs every 2 secs, and the level detector is also energized only during the time the source is pulsed; this reduces the likelihood of external light flashes affecting the device. In Fig. 1 the source is an LED, pulsed as above by a power supply P. The pulse from P also activates the threshold circuit L and operates switch S1 to disconnect that circuit's input from earth. The light emitted by the source is scattered by any smoke present to a detector C; the detector pulse passes via capacitor F to amplifier A and thence to the threshold circuit L. The capacitor prevents slow changes in ambient light being amplified. The threshold circuit compares the amplified pulse with a level pre-set by R2, and delivers a SET signal to flip-flop FF if that level is exceeded. At the start of the next power supply pulse the flip-flop is reset by a signal from differentiator D. The flip-flop can either activate an alarm K directly, or, as shown in Fig. 1, via an integrator T whose time constant is such that two consecutive pulses are required from the flip-flop before the alarm is activated. Power supplies: it is desirable to isolate the detector circuitry from the light source power supply. In Fig. 4 the power is supplied on line V1, and the pulse supply P activates a switch S2 as well as the light source and threshold circuit. During light emission the circuitry thus receives its power from a capacitor F1 which is recharged via R when S2 is closed at the end of each pulse. This use of a switch may be combined with or replaced by the use of smoothing chokes feeding capacitors F1, F2, as in Fig. 5 (not shown), or by resistors and Zener diodes feeding those capacitors, as in Fig. 6 (not shown).